Polyolefins

Polyolefin homopolymers are made from ethylene, propylene, butylene, and methyl pentene. Other olefin monomers such as pentene and hexene are used to make copolymers.

Because the chemical and electrical properties of all olefins are similar, they often compete for the same applications. They differ from each other primarily in their crystalline structure. However, since strength properties vary with the type and degree of crystallinity, the tensile, flexural, and impact strength of each polyolefm may be quite different. Stress-crack resistance and useful temperature range also vary with crystalline structure.

In addition to the solid polyolefin resins, these materials are also available as beads from which very low density (1.25 to 5.0 lb/ ft3) foam shapes and blocks are produced. Resilience and energy-absorption properties of these products are exceptional compared to those of conventional polystyrene foams. Polymers available as moldable beads include polyethylene, polypropylene, and a PE/polystyrene copolymer alloy.

The bead forms can be processed by the same methods used for expandable polystyrene (EPS). After the beads are expanded (20 to 40 times that of the solid resin) nd conditioned, they are poured into a mold and heated, usually by direct injection of steam. This softens, expands further, nd fuses the particles together, forming a niform, void-free, closed-cell shape. After olding, the shapes are usually annealed usually stored at 120 to 160°F to stabilize shape and dimensions.

Because they contain 80 to 95% air by volume, the foamed shapes are not nearly as strong as solid moldings. They are used priarily to cushion impact, insulate therally, and provide high stiffness-to-weight ore materials in composite components. important application for polypropylene am is in bumper cores for automobiles. A to 4-in.-thick section of foam at 2 to 4 b/ft3 can absorb the energy of a 5-mph imact. Package cushioning for fragile and aluable products such as electronic or auio components is another application for ither PE or PP foam. The toughness of E/PS alloy foams qualifies them for mateial-handling applications where repeated use is required.

The principal resins of the polyolefin family are polyethylene and polypropylene. Other polyolefin polymers and copolymers described here are ethylene-vinyl acetate, ionomer, polybutylene, and polymethyl pentene.

Polyethylene: The largest volume thermoplastic polymers used today, polyethylene is available in a variety of grades that have an equally wide range of properties. Some are flexible, others are rigid; some have low impact strength, others are nearly unbreakable; some have good clarity, others are opaque. Service temperatures can range from -40 to 200°F. In general, however, polyethylenes are characterized by toughness, excellent chemical resistance and electrical properties, low coefficient of friction, near-zero moisture absorption, and ease of processing.

Polyethylenes are classified according to density: low, medium, and high. A fourth type - ultrahigh-molecular-weight polyethylene (UHMWPE) - is in the medium-to-high density range. Properties of this high-performance plastic are entirely different from those of conventional polyethylenes. Also available is crosslinked polyethylene - a special grade, which, by chemical or irradiation treatment, becomes essentially a thermoset material with outstanding heat resistance and strength.

Low-density PE: LDPE, the first of the polyethylenes to be developed, has good toughness, flexibility, low-temperature impact resistance, clarity in film form and relatively low heat resistance. Like the higher density grades, LDPE has good resistance to chemical attack. At room temperature, it is insoluble in most organic solvents but is attacked by strong oxidizing acids. At higher temperatures, it becomes increasingly more susceptible to attack by aromatic, chlorinated. and aliphatic hydrocarbons.

Polyethylene is susceptible to environmental and some chemical stress cracking. Wetting agents such as detergents acceierate stress cracking. Some copolymers of LDPE are available with improved stresscrack resistance.

One of the fastest-growing plastics is linear low-density polyethylene (LLDPE), used mainly in film applications but also suitable for injection, rotational, and blow molding. Properties of LLDPE are different from those of conventional LDPE and HDPE in that impact, tear, and heat-seal strengths and environmental stress-crack resistance of LLDPE are significantly higher. Major uses at present are grocery bags, industrial trash bags, liners, and heavy-duty shipping bags for such products as plastic resin pellets.

High-density PE: Rigidity and tensile strength of the HDPE resins are considerably higher than those properties in the low and medium-density materials. Impact strength is slightly 1~wer, as is to be expected in a stiffer material, but values are high, especially at low temperatures, compared to those of many other thermoplastics.

HDPE resins are available with broad, intermediate, and narrow molecular-weight distribution. which provides a selection to meet specific performance requirements. As with the other polvethylene arades, very -high -molecular-weight copolymers of FIDPE resins are available with improved resistance to stress cracking.

Applications of HDPE range from film products to large, blow-molded industrial containers. The largest market area is in blow-molded containers for packaging milk, fruit juices, water, detergents. and household and industrial liquid products. Other major uses include high-quality, injection-molded housewares, industrial pails, food containers, and tote boxes; extruded water and gas-distribution pipe, and wire insulation; and structural-foam housings.

HDPE resins are also used to rotationally mold large, complex-shaped products such as fuel tanks, trash containers, dump carts, pallets, agricultural tanks, highway barriers, and water and waste tanks for recreational vehicles.

A special category of HDPE known as high-molecular-weight HDPE (HMWFIDPE) offers outstanding toughness and durability, particularly at low temperatures. These characteristics result from a unique combination of high average molecular weight (250,000 to 500,000), and a bimodal molecular-weight distribution.

In blow-molding applications, HMWHDPE allows drum manufacturers to meet DoT and OSRA specifications. In pipe production, HMW-HDPE meets PE-3408, currently the highest strength rating for PE pipe. Extruded sheet applications include pond liners, truck-bed liners, and outdoor leisure products. The primary market, however, for HMW-HDPE is in film applications, where its toughness allows downgauging in merchandise bags and trash bags. The material is also suited for use in T-shirt-type grocery sacks requiring high handle strength.

UHAMPE: Ultrahigh-molecular-weight polyethylene was originally defined as a polyethylene whose average molecular weight, as measured by the solution-viscosity method, is greater than 2,000,000. (Molecular weight of HDPE ranges from 100,000 to 500,000.) Over the years, producers and processors of UHMWPE materials have tried to reach agreement on just how high is "ultrahigh." Values proposed in the past have ranged from as low as 1,000,000 to over 3,500,000. Also in dispute was the question of the relationship between molecular weight and properties of UHMWPE in finished parts.

Several years ago, an ASTM task force agreed on the value of 3,100,000 molecular weight as the dividing line, above which the UHMW description should apply. Resin properties increase with increasing MW and start to level off at the 3,100,000 value. Also, processibility is more difficult above that dividing line, and material cost rises more rapidly. In 1981, the standard of 3,100,000 or higher for UHMWPE was approved by ASTM, and the final, full-society vote endorsed that recommendation, which is now defined in ASTM D4020.

As with most high-performance polymers, processing of UHMWPE is not easy. Because of its high melt viscosity (it does not register a melt-flow index), conventional molding and extrusion processes would break the long molecular chains that give the material its excellent properties. Methods used currently are compression molding, ram extrusion, and warm forging of extruded slugs. Developmental work is being done on injection molding of UHMWPE resins, but the process forces the polymer to behave in a manner that is not conducive to maintaining its molecular structure. Compression-molded sheets as large as 5 X 12 ft are available.

UHMW polyethylene has outstanding abrasion resistance and a low coefficient of friction. Impact strength is high, and chemical resistance is excellent. The material does not break in impact strength tests using standard notched specimens; doublenotched specimens break at 20 ft-lb/in. Crystalline melting point of the material is 267 ° F. Recommended maximum service temperature is about 200 ° F, however, because of a high coefficient of thermal expansion.

Polypropylene: Produced from propylene gas, polypropylene (PP) resins are sernitranslucent and milky white in color and have excellent colorability. Most PP parts are produced by injection molding, blow molding, or extrusion of either unmodified or reinforced compounds. Other applicable processes are structural-foam molding and solid-phase and hot-flow stamping of glass-reinforced sheet stock (a product of Azdel Inc., a joint venture of PPG Industries and General Electric Plastics).

Properties: Polypropylene is a low-density resin that offers a good balance of thermal, chemical, and electrical properties, along with moderate strength and moderate cost. Strength properties are increased significantly with glass-fiber reinforcement. Increased toughness is provided in special, high-molecular-weight, rubber-modified grades.

Electrical properties of PP moldings are affected to varying degrees by service temperature. Dielectric constant is essentially unchanged, but dielectric strength increases and volume resistivity decreases with increased temperature.

Polypropylene has limited heat resistance, but heat-stabilized grades are available for applications requiring prolonged use at elevated temperatures. Useful life of parts molded from such grades may be as long as five years at 250 °F, 10 years at 230°F, and 20 years at 210 °F. Specially stabilized grades are UL-rated at 120 °C (248 °F) for continuous service.

PP resins are unstable in the presence of oxidative conditions and UV radiation. Although all grades are stabilized to some extent, specific stabilization svstems are often used to suit a formulation for a particular environment. Polypropylenes resist chemical attack and staining and are unaffected by aqueous solutions of inorganic salts or mineral acids and bases, even at high temperatures. They are not attacked by most organic chemicals, and there is no solvent for the resin at room temperature. The resins are attacked, however, by halogens, fuming nitric acid and other active oxidizing agents, and by aromatic and chlorinated hydrocarbons at high temperatures.

Ethylene-vinyl acetate: EVA copolymers, which are derived from LDPE technology, are polyolefins that approach elastomeric materials in softness and flexibility. Yet, they can be injection, blow, compression, transfer, and rotationally molded; they are also extruded into film, sheeting, pipe, and profiles. Melt temperatures for these resins are generally 50 to 75°F lower than for polyethylenes. The resins can be pigmented to a broad range of colors, from pastels to deep hues.

EVA parts have good clarity and gloss, stress-crack resistance, barrier properties, low-temperature toughness, adhesive properties, and resistance to UV radiation. They have little or no odor, and they retain flexibility at low temperatures. Although their electrical properties are not as good as those of low-density polyethylene, EVA copolymers are competitive with rubber and vinyl products normally used for electrical applications.

The main limitation of EVA copolymers is their comparatively low resistance to heat and solvents. Chlorinated hydrocarbons, straight-chain paraffinic solvents, and benzene and its derivatives attack the resins.

They are not attacked, however, by alcohols, glycols, or weak organic acids. Heatdeflection temperature at 66 psi is about 145° F.

EVA copolymers are used principally in specialty applications, competing with plasticized PVC and rubber. FDA approves the use of these resins in direct contact with foods. Tubing made from EVA resi is used in medical equipment and in beveragevending, milk-packaging, and beer-dispensing equipment. Molded mechanical EVA parts include appliance bumpers, blow-molded bellows for air-operated toys, seals, and gaskets. The resins are also used in hot-melt adhesives.

lonomer: Interchain ionic bonding is what distinguishes ionomers from other polymers. These ionic crosslinks occur randomly between the long-chain molecules, producing properties that are usually associated with high-molecular-weight materials. At normal thermoplastic processing temperatures, however, the ionic bonding diminishes, allowing processing in co entional extruders and injection-molding, equipment.

Density of ionomers ranges from 0.94 to 0.97 gm/cm'. They are extremely tough, having tensile impact strengths as high as 600 ft-lb/in . and tensile strength as high as 5,000 psi, with elongation in the range of 300 to 500%. In addition, ionomers have excellent abrasion resistance (NBS Index as high as 640) and optical clarity (haze as low as 40%).

A compounded ionomer product is also available that is stiffer and has better heat resistance than the standard grades, and retains its excellent impact resistance. This product, intended for semirigid parts, resists many chemicals, solvents, greases, and oils.

The clarity, strength, and good adhesion of ionomer film to metal foils are responsible for its widespread use in food packaging, often as a heat-seal layer in composite structures. High impact strength and cut resistance have led to its use as bowling-pin and golf-ball covers. Automotive uses are based on impact toughness, light weight. and paintability. Foam injection-molded parts have replaced heavier rubber and metal bumpers guards and license-plate holders.

Footwear uses include box toes, heel counters, and injection-molded athletic soles to which metal cleats can be insert molded or spin welded. lonomer is used in ski boots and ice skates to provide lightweight durable shells. lonomer foams are used in ski-lift pads, boat bumpers, rnarine navigation buoys, and wrestling mats. Foamed sheet is used for thermal insulation on pipes and in covers for hot-water storage tanks.

Polybutylene: These resins are sernicrystalline thermoplastics manufactured in the U.S. by Shell Chemical Co- and sold under the trademark Duroflex. Compared to other Polyolefins, they have superior resistance to creep and stress cracking. Films made from polybutylene have high tear resistance, toughness, and flexibility. Chemical and electrical properties are similar to those of polyethylene and polypropylene, but the degree of crystallinity is much lower. This results in a rubberlike plastic that has very low molded-in stress. Rotationally molded polybutylene parts are virtually stress free.

The major applications for polybutylene resins are in pipe, packaging, hot-melt adhesives, and sealants. It is used as film for industrial refuse bags that resist bursting, puncturing, and tearing. Polybutylene pipe for cold-water applications has a higher burst strength than pipe made from any other polyolefin. Large-diameter pipe is used in mining and power-generation systems to convey abrasive materials.

Polymethylpentene: Major advantages of polymethylpentene (PMP) over other polyolefins are transparency in thick sections, short-time heat resistance to 400 ° F, and lower specific gravity. A moderately crystalline plastic, PMP is transparent because, unlike other polyolefins, its crystalline and amorphous phases have the same index of refraction. Almost optically clear, it has a light-transmission value of 90%, just slightly less than acrylic.

Polymethylpentene retains most of its physical properties in brief exposure to 400*F, but is not stable at temperatures over 300'F without antioxidant additives. In clear form, it is not recommended for longterm exposure to LN environments.

Chemical resistance and electrical properties of PMP are similar to those of other polyolefins except that it retains these properties at higher temperatures than either polyethylene or polypropylene. In this respect, it compares favorably with PTFE up to 300'F. Parts molded from the resin are hard and shiny, yet impact strength is high at temperatures down to -20'F. Specific gravity (0.83) is the lowest of any commercial solid plastic.

A major use of polymethylpentene is in molded containers for foods that are quickfrozen, then heated later in the same container. Other commercial applications Include bottle closures, hot-liquid level indicators, transparent plumbing systems, coffeemaker bowls, medical syringes, laboratory ware, and light diffusers. Electrical/electronic applications include wire coatings and components for microwave equipment.

Polydicyclopentadiene: PDCPD is a thermoset polymer formed by the metathesis polymerization of dicyclopentadiene (DCPD). DCPD components can be used in reaction injection molding, structural RIM, resin-transfer molding, casting, and other liquid-molding processes.

In these processes, low viscosity (less than 350 centipoises) A and B components are mixed at a 1:1 ratio and Pumped into a closed mold at pressure less than 50 psi. The Polymerization is carried out in the mold, forming a rigid, high-impat, unreinforced plastic. Fillers can be included in the components to enhance Polymer rigidity or lower the coefficient of linear thermal expansion. When continuous glass mat reinforcement is preplaced in the mold before injection, as in the SRIM and RTM processes, the resulting composite formed has structural properties.

PDCPD has low density and a unique balance of rigidity and impact while being resistant to chemical and having a hydrophobic character. The creep resistance is typical of a thermoset polymer when it is utilized in a temperature range which is best characterized by the heat deflection temperature of 264 psi at 100 to 115°C, depending on formulation. PDCPD develops a thin oxidized layer when exposed to oxidizing or LTV conditions ' which inhibit further oxidation by acting as a barrier to oxygen. All grades are stabilized for specific environmental requirements. Parts requiring longterm outdoor exposure can be painted with the surface readily accepting most paint systems.